db: Remove column_family stub

There's a proper column_family in database.hh now. Remove a stub that
was introduced during the initial conversion.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

db/column_family.hh

@@ -1,12 +0,0 @@
-#ifndef DB_COLUMN_FAMILY_HH
-#define DB_COLUMN_FAMILY_HH
-
-namespace db {
-
-class column_family {
-    // FIXME: stub
-};
-
-}
-
-#endif